1. Field of the Invention
The present invention relates to a punch die which can form through-holes with a high aspect ratio at a high precision and high density. More specifically, the present invention relates to a punch die for simultaneously punching and laminating a plurality of thin sheets of a material to be processed usable for the method of laminating a punched material in a punch. Using the punch die of the present invention, it is possible to form many through-holes through several sheets of a material laminated to a prescribed thickness with a high aspect ratio at the same high precision as in the case of making holes through one thin sheet of material, even if the material is so soft that the punched material may deform during handling after punching.
2. Description of the Background Art
All industrial products should be cheaper, lighter, and smaller. Minuteness is very important to increase an added value particularly for industrial parts with a number of electronic circuits mounted thereon. Development of mounting technologies backing up the industry is remarkable. For example, printed circuit boards on which electronic parts are mounted must have minute through-holes accurately drilled therethrough to ensure formation of high-density circuits while maintaining high reliability. Circuits for printed circuit boards are formed by making through-holes in substrates and inserting lead electrodes or burying conductive bodies in the through-holes. Such circuits have a risk of losing reliability due to short circuits and the like that may occur during use, if these circuits are not highly integrated or do not have high precision. Therefore, the technology of making through-holes with high precision is essential for industrial parts.
In recent years, along with the development of high integration technologies it has become necessary to make many minute through-holes in a given area of a printed circuit board. As a matter of course, such holes must be small and deep. Specifically, the holes must have a small diameter and a long axis, or, in other words, must have a high aspect ratio. It is necessary to form such through-holes at a high precision. Generally, when a through-hole is cylindrical in shape, the aspect ratio indicates a ratio of the diameter to the axis length. When a through-hole is not a cylinder, the aspect ratio indicates a ratio of the minimum distance from one position to the opposing position on the periphery of the opening of the through-hole to the axis length. Here, the minimum distance from one position to the opposing position on the periphery of the opening of a through-hole indicates a minimum distance S in FIGS. 5(a) and 5(b). Specifically, a through-hole with a high aspect ratio is a long narrow hole having a longer axis length as compared with the diameter or the minimum distance from one position to the opposing position on the periphery of the opening of the through-hole.
A stamping machine is popularly used for making small through-holes in such a material to be processed. The stamping machine is an apparatus having a punching die consisting of a punch and a die. The machine can make many holes at one time by pressing a material to be processed using such a punching die. Construction and operation of a well-known stamping machine will now be described taking a die as an example.
FIG. 4 is a side view showing an example of a punch die provided in a conventional stamping machine. A conventional punch die 141 provided in a stamping machine 140 consists of an upper portion of die 25 and a lower portion of die 26. The upper portion of die 25 is composed of a punch 10, a punch holder 47 holding the punch 10, a stripper 11 positioning the tip of the punch 10, and a die set (upper) 44. The lower portion of die 26 is composed of a die 12 and a die set (lower) 54 holding the die 12. The die set (upper) 44 is composed of a punch base 42 and a punch plate 43, whereas the die set (lower) 54 is composed of a die base 52 and a die plate 53.
The die 12 in which a spot 21 is formed is secured with the die set (lower) 54. The punch 10 is secured with the die set (upper) 44. The integrated punch 10/die set (upper) 44 runs along the guide post 29 for the upper portion of die, standing on the die set (lower) 54, and is brought down onto the die 12 by the pressure of a press machine 27. A material (not shown) to be processed placed on the die 12 is punched by the punch 10 descending from above the die 12, producing a punch hole therein.
When the punch 10 is drawn upwards the stripper 11 plays a role of holding the processed material and removing it from the punch 10 so that the material may not move upward together with the punch 10. The stripper 11 is suspendingly secured with a punch holder 47 and a stripper-suspending bolt 48, and pushed down by a stripper spring 49 inserted between the punch plate 43 and the stripper-suspending bolt 48. Therefore, the stripper 11 can freely move vertically with respect to the punch holder 47 to a predetermined extent. When the punch 10 is operated, specifically when a subguide post 28 descends along a guidepost hole in the die 12 along with downward movement of the upper portion of die 25, the stripper 11 guides the punch 10 and, at the same time, securely holds the material to be processed on the die 12 by its surface.
When the punch 10 punches the die 12 together with the material to be processed, the stripper 11 moves upward with respect to the punch 10 due to contraction of the stripper spring 49. Therefore, the punching operation of the punch 10 is not restricted. When the punch 10 is drawn up, the stripper 11 moves downward with respect to the punch 10 due to elongation of the stripper spring 49. This allows the punch 10 to be housed in the stripper 11 and also allows the upper portion of die 25 to return to the position before the punching operation.
A punching operation on a thick material using this type of conventional punch die is subject to a problem of insufficient accuracy due to the requirement of a wide clearance between the punch and die, if the thick material itself is a target of the punching operation. In addition, a thick material receives a larger shearing force than a thin material upon punching. When a through-hole density is large, the die must be provided with many holes. Therefore, the die may not withstand the large shearing force, resulting in deformation due to lack of rigidity. In a worse case, the die may fracture.
FIGS. 7(a) and 7(b) show a hole in a through-hole section made by punching using a conventional punch die. As shown in FIG. 7(a), when a material to be processed 13 placed on the die 12 is punched by a punch 10 with a clearance 16 between the punch 10 and the die 12, cracks 15 are produced at edges 14 of the punch 10 and die 12. These cracks 15 are produced within the width of the clearance 16. The accuracy of the through-holes fluctuates within the width of the clearance 16. As a result, when a through-hole is produced using a punch die, the cross-section of the through-hole through the processed material after punching generally has a tapered (downwardly expanding) configuration as shown in FIG. 7(b).
According to an article described in “Basic Machine Work (I)” published by The Nikkan Kogyo Shimbun Co., Ltd., the clearance 16 required for a punch die is 4-12% of the thickness for a thin plate and 18-26% of the thickness for a thick plate. The thicker the plate, the larger the clearance must be. Specifically, as mentioned above, the thicker the processed material, the lower is the precision of the through-hole. As a result, the punched holes fluctuate in size. It is difficult to produce a through-hole with a high aspect ratio and a small diameter at high precision.
A product with a desired thickness may be produced using the same type of conventional punch die. According to this method, a number of thin sheets of material to be processed are punched instead of punching a thick plate at one time, and the punched sheets are transferred and laminated to produce a product with the target thickness. When a hole is made using this method, each sheet is provided with a hole with a high precision because the sheet processed in each punching operation is thin. In addition, a shearing force produced by the punch and die is small, enabling the stamping machine to produce holes at a high density. However, the method requires not only a jig to transfer the processed sheets of the material and a space for laminating the punched sheets, but also additional process steps, resulting in lower productivity and high cost. In addition, a guide pin to precisely laminate the punched sheets is necessary, which requires an additional step of opening otherwise unnecessary holes through the material to be processed. Moreover, when a soft material that may be deformed after punching is used, the position of the holes may deviate while the punched sheets are transferred for lamination. The resulting laminated material with a prescribed thickness may have through-holes with impaired precision. Therefore, it is difficult to produce through-holes with a high aspect ratio and a small diameter at a high density using this method.
As a method of making through-holes without using a punch die, a hole-opening process using laser beams is known. This is a device for processing the material by focusing laser beams onto the object to be processed. Focusing laser beams employed as a principle of the laser beam processing inevitably produces through-holes tapering in the direction of the laser beams. The process also has a fundamental problem of producing only inaccurate holes if the target through-holes have a high aspect ratio.
FIGS. 8(a) and 8(b) show a boring operation using laser beams. In a laser beam machine shown in FIG. 8(a), parallel beams 17 passing through a condensing lens 18 are converged at a focal distance 20 to make a hole. The larger the distance from the focal point, the larger are the width 19 of laser beams and the diameter of the through-holes to be produced. Therefore, the thicker the sheet of the material to be processed, the larger is the diameter of the through-hole produced in the laser beam inlet side when the boring operation is carried out on the laser beam exit side. As a result, a through-hole with an upwardly tapered configuration as shown in FIG. 8(b) is formed.
In addition, since heat energy is used in the laser beam process, the processed material is deformed with heat. This result in formation of denatured layers, which causes another problem of fluctuation in the through-hole diameters. Also with respect to this problem, the thicker the material, the larger is the amount of laser beams (or heat energy) required. Therefore, the thicker the processed material, the lower is the accuracy of the through-holes. For these reasons, it is also difficult to produce through-holes with a high aspect ratio and a small diameter at a high density using a laser process machine.
As mentioned above, since mounting technology in the industrial field, especially in the field of electronic parts, has become highly integrated, industrial parts such as interconnecting substrates for mounting electronic parts are required to be provided with minute through-holes at a high density. A device for forming through-holes with a high aspect ratio at a high precision without producing damage, even when the materials used for such industrial parts are so soft that the industrial parts may be deformed in terms of both dimension and shape according to handling after processing, has been required. However, conventional through-hole making processes using a punch die or laser beams do not satisfy this requirement.
To satisfy such a requirement, the inventors of the present invention have conducted extensive studies on hole making techniques using a punch die. As a result, the inventors have found that that a method of punching holes in a material to be processed using a punch die consisting of a punch and a die and, at the same time, laminating the material without drawing it from the punch is effective in obtaining a processed material with a prescribed thickness. More specifically, after making a hole through the material to be processed, that material is caused to adhere to a stripper and raised from the die without being drawn out from the punch. Then, the punch is returned in such a manner that it is slightly drawn out from the lowest part of the hole formed in the material. The next material is processed in the same manner. Specifically, after making a hole using a punch, the material is caused to adhere to the bottom of the previously processed material and raised from the die without being drawn out from the die. Then, the punch is returned in such a manner that it is slightly drawn out from the lowest part of the hole formed in the raised material. The laminated material with a prescribed thickness obtained by repeating this procedure was found to have through-holes with a high aspect ratio and a small diameter formed therein at a high precision.
Since the punch is not drawn out from the hole of the processed material up to the completion of punching operation, during which several sheets of processed material are laminated using the punch as a lamination axis in the punch die, deformation of the hole can be prevented. In addition, because the punch is secured to the processed material, there should be no changes between the center axis of the punch and the center axis of the stripper hole in terms of both the direction of deviation and the size. Therefore, it is possible to make a through-hole with a high precision in a processed material having a prescribed thickness. The through-hole boring at a high precision makes it possible to produce a high-density through-hole section. The processed material can be suitably used as an industrial part such as an electronic part required for a highly developed mounting technology.
In the conventional punch die typified by the punch die 141 provided in the stamping machine 140 shown in the FIG. 4, although the stripper 11 is free with respect to the punch 10 and the die set (upper) 44, the positions of the punch 10 and the stripper 11 are fixed when the punch 10 is drawn up and the punch 10 is usually stored in the stripper 11. Therefore, the operation of holding the punch without drawing it out from the hole of the processed material up to the completion of the punching operation, while laminating several sheets of processed material using the punch 10 as a lamination axis is actually impossible.
An additional problem with a conventional punch die is the low precision of the through-hole if the processed material is soft and easily deformed. In the punching operation, before the punch 10 makes a hole through the material to be processed, the stripper 11 hits the material to be processed placed on the die 12, adding a compressive force to the material. Since the hole is opened by the punch 10 with the processed material placed on the die 12 being pressed due to the compressive force, the hole precision is impaired with respect to both the location and shape to the extent of form-return of elastic deformation after punching.
Therefore, the punch die for forming a through-hole with a high aspect ratio and small diameter at a high precision in a material to be processed must be provided with functions which have not been possessed by conventional punch dies. For example, capabilities such as laminating several sheets of processed material using the punch as a laminating axis, while changing the length of the punch protruding from the bottom surface of the stripper when the punch is raised each time the material is punched, the capability of suppressing generation of a compressive force in a manner whereby the stripper or the processed materials through which holes have been opened laminated in the punch may not directly come onto the processed material during the punching operation, and the like.
The present invention has been achieved in view of the above problems and requirements. An object of the present invention is therefore to provide a punch die suitable for a simultaneous punching-lamination process which can open extremely small through-holes almost straight, having a diameter of 100 μm or less, for example, and a ratio of the axis length to the diameter larger than a prescribed value, and a high precision equivalent to holes made through a sheet of thin material, even if the material to be processed is soft and deformable. Industrial parts having a specified thickness and many through-holes with a high aspect ratio and small diameter can be produced using the punch die, thereby contributing to increased integration in the mounting technology for industrial parts, including electronic equipment as a major product.